A variety of devices has been constructed to create a thin flowing laminar fluid film. Falling film evaporators are a common example. An evaporator is a device designed to convert a liquid into a gas. Evaporation may be desired for various reasons, including for the distillation or purification of a liquid such as water. Seawater may be converted into potable water using evaporative technology, and this application is particularly important given the growing demand for potable water.
In a falling film evaporator, contaminated liquid is supplied to generally vertical heat exchange tubes. For example, in a seawater purification evaporator of this type, the seawater is the input. Seawater flows down vertical tubes while heat—typically in the form of steam—is supplied to the area outside the tubes. By allowing only a thin film of seawater to flow down the tubes, the heat transferred to the water is sufficient to evaporate some of the water. This water vapor, which is now pure water, rises up the center part of the tube. The vapor is then collected in some fashion and condensed to produce pure water.
Falling film evaporators work best when the flowing film thickness is maintained at a desired thickness. Returning to the seawater evaporator example, a relatively thin film is desired so that maximum evaporation will occur. If the film is too thick, evaporation will be inhibited. If the film is too thin, all the water will evaporate, leaving the tube surface dry. The latter situation can be damaging because of the combination of the high temperature tube surface with the various salts and other contaminants left behind by the evaporation. Hard scale deposits can result, and such scale can be effectively baked onto the tubes. This scale can be hard to remove, can reduce the heat transfer capability of the tubes, and can cause localized tube corrosion, particularly where the deposited materials are corrosive. For this reason, it is important to prevent drying of the tube surfaces in a falling film seawater evaporator.
One means used to avoid drying of the tube surface is to increase the flow rate of the fluid. By flowing more seawater down the tubes, there is less risk of the tubes drying. However, this also means the film layer will be thicker, which tends to reduce the evaporation rate. To offset this reduced thermal efficiency, a seawater falling film evaporator may employ a higher temperature. That is, rather than heating the tubes to 140° F., which is considered an optimal temperature for evaporation without significant scale adhesion to the heat transfer surface, an evaporator using higher seawater flow rates may need to raise the temperature substantially above this point. That increased heating will produce more evaporation, but it also will result in more baked on scale on the tubes. These trade offs render the falling film evaporator much less desirable as a means for purifying seawater.
In some applications, it may be desirable to evaporate the flowing fluid quickly within a particular region of the apparatus. For example, some liquids may contain certain entrained or dissolved gasses with relatively low flash points. It may be desired to selectively evaporate off these materials so that each can be separately handled. This operation is difficult to achieve in a conventional falling film evaporator. To achieve this result, long tubes may be needed and separate heating regions used along the tubes.
A somewhat new use of flowing film technology involves use of thin films of algae to capture carbon dioxide from the atmosphere. Algae use light and carbon dioxide to create energy, oxygen, and other products. Algae have been identified as a potentially important means of capturing carbon, that is, by removing carbon dioxide from the air.
In a falling film algae carbon capture apparatus, a thin film of algae may flow through an area exposed to light. Carbon dioxide rich air may be fed into the apparatus in the lighted region. The carbon dioxide is consumed by the algae which then flows past the lighted region and is further processed to remove any desired by products of the operation. For this process to work well, it is important to have the algae at a warm temperature, and that means some evaporation will occur. It is also desirable to maintain the algae at a constant film thickness to better facilitate the carbon capture.
These potential uses of falling film technology and the inherent trade offs presented by the use of a falling film evaporator to purify seawater or other contaminated liquids identify an important need. There is a need for an improved falling film apparatus and method that will obtain the potential benefits without the undesirable consequences. The present invention provides just such an apparatus and method.